This invention relates to a process for forming a mask for use in ion milling magnetic material, and particularly for use in ion milling the shape of poles in thin film magnetic heads.
Thin film heads employ top and bottom pole pieces separated at the air-bearing surface by a thin layer of insulating material forming the gap. Portions of a coil pass between the magnetic poles distal from the air-bearing surface. The coil portions are separated from the magnetic poles by insulating material.
Reference is made to U.S. application Ser. No. 08/331,684 filed Oct. 31, 1994 for "Thin Film Head Design Improving Top Pole Width Control" by Stageberg et at. and assigned to the same assignee as the present application, which is incorporated herein by reference. The Stageberg et at. application describes the problem of formation of "hills" between the coil region and the air-bearing surface in thin film heads. The "hill" is the result of the coils forming a thicker portion to the head. Certain layers employed in the processing of the head are formed by a spin coating process whereby the layer of material is applied to the wafer in which the head (and others) is being formed and the wafer is spun to cause the material to spread out across the wafer. However, as explained in the Stageberg et at. application, the presence of the hill on the head causes the spun layer to be applied unevenly to the head. One example of a spin-coated layer is the photoresist mask used to pattern the desired shape of the pole in an ion milling process. A patterned photoresist is applied to the top pole of each head, and the wafer (and heads) is ion milled to simultaneously remove photoresist and unwanted magnetic material, resulting in the pole shaped to the desired configuration. It is important that the photoresist layer have an even thickness over the shape being milled. However, the photoresist, being applied by a spin coating process, is thinner at the hill than at other portions of the mask. As a result, a risk exists that the photoresist will be completely milled away at the hill region, resulting in unwanted milling of the top pole material and deterioration of the top pole.
Some success has been accomplished using metal masks. Metal is deposited onto the top pole and a photoresist is applied to the metal layer by a spin coating process. The photoresist is then patterned to the desired pole shape. The metal is etched to finish the mask, usually with a wet etchant, by argon ion milling or by reactive ion etching using a fluoride. One advantage of the metal mask is that the metal has a mill rate of about one-half that of magnetic material (compared to photoresist which has a mill rate about equal to that of magnetic material). Consequently, the metal forming the ion milling mask need only be about one-half the thickness of a corresponding photoresist mask. Since the metal mask is milled at a rate slower than the magnetic material, greater control can be achieved over the milling conditions than can be accomplished using photoresist masks. Another advantage of metal masks is that they are evenly deposited onto the pole material, rather than spin coated, thereby avoiding the disadvantages of spin coating.
However, metal masks are not without problems. Particularly, if the metal is patterned or etched using a wet etchant which also attacks the magnetic material forming the top pole, the risk exists that if the process of forming the metal mask is not precisely controlled, the top pole may be etched, resulting in deterioration of the top pole. There is, accordingly, a need for a mask providing greater control over mask thickness for purposes of ion milling the top pole of a thin film head.